I-RT-36, an aromatic tris-amidine compound, has been demonstrated to inhibit porcine pancreatic and human plasma kallikrein, with a Ki in the 10 to the negative 8th power range. The purpose of the present study was to evaluate this inhibitory capacity both in vivo and in vitro using the rat as an experimental model. Initial studies demonstrated blockade of the expected hypotensive response provoked by the injection of kallikrein isolated from rat urine, when the experimental animal was pre-treated with the I-RT compound. In work with guinea pig ileum, addition of the tris-amidine to the bath containing a kininogen source and kallikrein blocked the expected kinin induced smooth muscle contraction.